Throughout this application, various publications, patents and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
An electroactive material that has been fabricated into a structure for use in a battery is referred to as an electrode. Of a pair of electrodes used in a battery, herein referred to as an electric current producing cell, the electrode on the electrochemically higher potential side is referred to as the positive electrode, or the cathode, while the electrode on the electrochemically lower potential side is referred to as the negative electrode, or the anode.
An electrochemically active material used in the cathode or positive electrode is referred to hereinafter as a cathode active material. An electrochemically active material used in the anode or negative electrode is hereinafter referred to as an anode active material. An electric current producing cell or battery comprising a cathode with the cathode active material in an oxidized state and an anode with the anode active material in a reduced state is referred to as being in a charged state. Accordingly, an electric current producing cell comprising a cathode with the cathode active material in a reduced state, and an anode with the anode active material in an oxidized state, is referred to as being in a discharged state.
As the evolution of batteries continues, and particularly as lithium batteries become more widely accepted for a variety of uses, the need for safe, long lasting, high energy batteries becomes more important. There has been considerable interest in recent years in developing high energy density cathode-active materials and alkali metals as anode active materials for high energy primary and secondary batteries.
To achieve high capacity in electric current producing cells or batteries, it is desirable to have a high quantity or loading of electroactive material in the cathode layer. For example, the volume of cathode coating layer in an AA size battery is typically about 2 cm.sup.3. If thc specific capacity of the electroactive material is 1000 mAh/g, the amount or volumetric density of the electroactive material in the cathode coating layer would need to be at least 500 mg/cm.sup.3 in order to have the 1 gram of cathode active material in the AA size battery necessary to provide a capacity of 1000 mAh or 1 Ah. If the volumetric density of electroactive material in the cathode coating layer can be increased to higher levels, such as greater than 700 mg/cm.sup.3, the capacity of the battery can be proportionately increased to higher levels.
There are a wide variety of electroactive materials that are utilized in the cathode layers of electric current producing cells. For example, a number of these are described in copending U.S. patent application Ser. No. 08/859,996, titled "Novel Composite Cathodes, Electrochemical Cells Comprising Novel Composite Cathodes, and Processes for Fabricating Same" by common assignee. These electroactive materials vary widely in their specific densities and in their specific capacities so the desired volumetric densities in mg/cm.sup.3 correspondingly vary over a wide range. Lithium and sulfur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, of electric current producing cells because they provide nearly the highest energy density possible on a weight or volume basis of any of the known combinations of active materials. To obtain high energy densities, the lithium can be present as the pure metal, in an alloy, or in an intercalated form, and the sulfur can be present as elemental sulfur or as a component in an organic or inorganic material with a high sulfur content, preferably above 50 weight per cent sulfur. For example, in combination with a lithium anode, elemental sulfur has a theoretical specific capacity of 1680 mAh/g, and carbon-sulfur polymer materials with trisulfide and longer polysulfide groups in the polymer have shown specific capacities of 1200 mAh/g. These high specific capacities are particularly desirable for applications, such as portable electronic devices and electric vehicles, where low weight of the battery is important.
Herein, the term "carbon-sulfur polymer materials" means materials comprising carbon-sulfur polymers with carbon-sulfur single bonds and with sulfur-sulfur bonds forming trisulfide (-SSS-) and higher polysulfide linkages. These carbon-sulfur polymer materials comprise, in their oxidized state, a polysulfide moiety of the formula, -S.sub.m -, wherein m is an integer equal to or greater than 3. For example, these carbon-sulfur polymer materials are described in U.S. Pat. Nos. 5,601,947; 5,609,702; 5,529,860; and in copending U.S. patent application Ser. No. 08/602,323 to Skotheim et al., now abandoned. Organo-sulfur materials with only disulfide groups typically show specific capacities in the range of 300 to 700 mAh/g and are accordingly less desirable for those applications requiring high specific capacities.
It is well known to those skilled in the art of battery design and fabrication that practical battery cells comprising the electroactive cathode and anode materials also contain other non-electroactive materials such as a container, current collectors, electrode separators, polymeric binders, conductive additives and other additives in the electrodes, and an electrolyte (typically an aqueous or non-aqueous liquid, gel, or solid material containing dissolved salts or ionic compounds with good ionic conductance but poor electronic conductivity). All of these additional non-electroactive components are typically required to make the battery perform efficiently, but they also serve to reduce the gravimetric and volumetric energy density of the cell. It is, therefore, desirable to keep the quantities of these non-electroactive materials to a minimum so as to maximize the amount of electroactive material in the battery cell.
To achieve the highest possible volumetric density of the electroactive material in the cathode coating layer, it is desirable to maximize the weight per cent for electroactive materials in the coating layer, for example, 65 to 85 weight per cent for electroactive materials of a specific density of 2 g/cm.sup.3, and to maintain the porosity or air voids in the cathode coating layer as low as possible, for example, 40 to 60 volume percent. Particularly, the porosity of the cathode coating layer must be kept low because higher porosities, for example, 70 to 85 volume per cent, do not provide enough electroactive material to obtain very high cell capacities. With the electroactive transition metal oxides, this is often relatively easy to achieve because these oxides typically have electrically conductive properties and are typically microporous so that high levels of added conductive fillers and microporous additives are not required. With electroactive sulfur-based compounds, which have much higher specific capacities than the electroactive transition metal oxides, it is difficult to obtain efficient electrochemical utilization of the sulfur-based compounds at high volumetric densities because the sulfur-based compounds are highly electrically non-conducting or insulative and are generally not microporous. For example, U.S. Pat. No. 5,532,077 to Chu describes the problems of overcoming the insulating character of elemental sulfur in composite cathodes and the use of a preferably homogeneous distribution of an electronically conductive material, such as carbon black, and of an ionically conductive material in the composite electrode to try to overcome these problems.
To overcome these limitations with electroactive sulfur-based compounds, large amounts of electrically conductive fillers, such as conductive carbons, are typically added to the cathode coating layer. However, the formation of insulating sulfur coatings on the conductive carbon and current collector has been observed which isolates these components from the rest of the cell components and leads to poor electrochemical capacity and cycling. U.S. Pat. No. 3,639,174 to Kegelman describes composite cathodes comprising elemental sulfur and a particulate electrical conductor. U.S. Pat. No. 4,303,748 to Armand et al. describes composite cathodes containing an ionically conductive polymer electrolyte together with elemental sulfur, transition metal salts, or other cathode active materials for use with lithium or other anode active materials in which, for example, the active sulfur or other cathode active material and the inert compounds with electrical conduction, such as graphite powder, are both particles of between 1 to 500 microns in diameter. U.S. Pat. No. 5,162,175 to Visco et al. describes the use of 1 to 20 weight percent of particles, such as carbon black, to provide electrical conductivity in solid composite organo-sulfur cathodes containing electroactive disulfide materials. U.S. Pat. No. 5,460,905 to Skotheim describes the use of p-doped conjugated polymers, together with an effective amount of conductive carbon pigments, for the transport of electrons in sulfur-based cathodes. U.S. Pat. No. 4,375,427 to Miller et al. describes conductive polymers for use in composite cathodes where the conductive polymers are reinforced with glass, asbestos, or metal fibers or with conductive carbon or graphite pigments and where the conductive polymers are in the form of a fiber. U.S. Pat. No. 5,324,599 to Oyama et al. describes the use of a combination of a compound having an electroactive disulfide group and a conductive polymer, preferably having a porous fibril structure, for organo-sulfur composite cathodes. U.S. Pat. No. 5,529,860 and U.S. patent application Ser. No. 08/602,323 to Skotheim et al. describe the use of conductive carbons and graphites, conductive polymers, and metal fibers, powders, and flakes as conductive fillers with carbon-sulfur polymer materials.
Another approach to obtain electrical conductivity with sulfur-based cathodes is to introduce metallic conductivity into the normally insulating sulfur-based material. U.S. Pat. No. 5,516,598 to Visco et al. discloses composite cathodes comprising metal-organosulfur charge transfer materials with one or more metal-sulfur bonds, wherein the oxidation state of the metal is changed in charging and discharging the positive electrode or cathode. The metal ion provides electrical conductivity to the material, although it significantly lowers the cathode energy density and capacity per unit weight of the polyorgano-disulfide material and does not enhance the microporosity of the cathode coating layer.
It would be advantageous to significantly increase the volumetric densities of electroactive sulfur-based cathodes without sacrificing the high specific capacity of these materials, i.e., without reducing the high electrochemical utilization during cycling of the cells. Particularly as the cathode coating thickness is increased, it becomes progressively more difficult to achieve the electrical conductivity and microporosity needed for highly efficient utilization of the sulfur-based materials. For example, it would be very beneficial to the capacity of the batteries if the volumetric density of cathode active material could be increased by 50% or 100%. One method to increase the volumetric density of the cathode coating is by compressing or calendering the coating layer to a reduced thickness. It would be very advantageous to be able to compress or calender the cathode coating layer to a 20% or greater reduction in thickness without sacrificing the high specific capacity of the electroactive sulfur-based materials.
With electroactive sulfur-based compounds, higher volumetric density through reduced porosity of the cathode coating layer is typically useful in suppressing the out-diffusion of reduced sulfide compounds from the cathode during cycling of the battery. This contributes to a lower rate of loss of capacity during cycling of the battery.
Japanese Patent Publication No. 09-147868, published Jun. 6, 1997, describes the use of active carbon fibers to absorb electroactive sulftir compounds in cathodes of secondary batteries and to provide increased cycle life at high discharge currents. These active carbon fibers are characterized by highly microporous structures with specific surface areas above 1000 m.sup.2 /g, which absorb large amounts of sulfur compounds, such as 30 to 50 weight per cent, into the pores. These active carbon fibers also have diameters greater than 1 micron, typically in the range of 2 to 6 microns. There is no mention of non-activated carbon filaments or nanofibers with submicron diameters.
The use of fibers of different types in solid cathodes not containing sulfur-based active materials has been reported. For example, J Power Sources, 1996, 58, 41-54 by Frysz et al. describes the use of carbon filaments or nanofibers to provide improved cathode absorptivity to electrolytes, compressibility and packing density, bindability and mechanical strength, and reduced polarization when the carbon filaments are substituted for carbon black in MnO.sub.2 cathodes with lithium anodes. In Mat. Res. Soc. Symp. Proc. 1995, 393, 367-371, by Frysz et al., the use of submicron carbon filaments in place of carbon black as porous reduction electrodes in carbon limited lithium batteries with bromine chloride in thionyl chloride catholyte is described. U.S. Pat. No. 5,514,496 to Mishima et al. describes the use of a conducting agent of carbon or metallic fibers and of a filler of a fibrous material undergoing no chemical change, such as fibers of polyolefins, glass, and carbon. Also, for example, U.S. Pat. No. 5,437,943 to Fujii describes the use of electroconductive auxiliary agents, such as carbon or metal fibers, to provide voids in a cathode containing electroactive vanadium oxide and a conducting polymer. The purpose of the voids is to hold electrolyte components. U.S. Pat. Nos. 5,032,473 to Hoge and 5,053,375 to Rao disclose the use of a web of conductive carbon fibers with carbon particles or silver-adsorbed carbon particles in metal/air batteries. U.S. Pat. No. 5,225,296 to Ohsawa et al. discloses an electrode of a porous carbon sheet of carbon fibers and carbon particles. U.S. Pat. No. 5,451,476 to Josefowicz describes a composite carbon fiber and electrically conductive polymer cathode where the conductive polymer is on the surface of carbon fibers, typically about 1 micron in diameter. U.S. Pat. No. 4,940,524 to Perineau et al. discloses the use of carbon or graphite fibers in cathodes containing a Raney metal.
Another approach to provide the necessary electrical conductivity with solid sulfur-based cathodes has been to impregnate or coat a carbon or graphite felt material with the sulftir-based electroactive material. Often, the conductive felt material has the function of the current collector that provides good electrical conductivity between the positive electrode and a metal container, as, for example, described in U.S. Pat. No. 5,516,598 to Visco et al. Typically, the felt material is a current collector substrate to the cathode coating layer or is an intermediate matrix layer between the cathode coating layer and the current collector and, as such, the conductive fibers do not extend tlroughout the cathode layer. U.S. Pat. No. 5,542,163 to Chang describes the use of carbon in the form of powder, flakes, beads, or fibers as an adhesion promoting layer between a cathode and a current collector. The carbon or graphite fibers in these various felt materials have physical dimensions, such as diameters greater than 1 micron, similar to those of the active carbon fibers described above for Japanese Patent Publication No. 09-147868, published Jun. 6, 1997. U.S. Pat. No. 3,532,543 to Nole et al. describes the use of a porous carbon cloth structure to provide enough porosity to allow good electrolyte penetration into cathodes with elemental sulfur as the active material, preferably in a mixture with a particulate conductive carbon and a suitable binder. U.S. Pat. No. 5,506,072 to Griffin et al. describes a woven graphite sheet electrode packed around with large particles of elemental sulfur and conductive graphite pigments. U.S. Pat. No. 3,915,743 to Lauck discloses the use of 2 to 10 per cent by weight of asbestos fibers in composite cathodes of elemental sulfur and conductive carbon or graphite powder. The asbestos fiber is stated to increase the porosity of the sulfur electrode. However, asbestos fibers are not electrically conductive and have safety and environmental limitations.
U.S. Pat. No. 4,833,048 to Dejonghe et al. describes an electronically conductive matrix of carbon or aluminum fibers, preferably graphite felt, dispersed throughout a liquid organo-sulfur cathode having disulfide electroactive materials to provide good electrical conductivity between the positive electrode and the metal container. U.S. Pat. No. 4,945,013 to Lim describes the use of a graphite fiber matrix for the liquid sodium polysulfide cathode of a sodium-sulfur battery. In a sodium-sulfur cell, both the sulfur cathode and the sodium anode are liquid at the operating temperatures of the cell. U.S. Pat. No. 5,246,794 to Blomgren et al. describes a cathode current collector of carbon fibers for use with liquid cathodes where the carbon fibers have a diameter of 0.2 microns or less. Liquid sulfur-based cathodes are typically electrochemically irreversible and not practical for use in rechargeable cells. Liquid sulfur-based cathodes are substantially different from solid sulfur-based cathodes in a number of other ways in addition to electrochemical reversibility and, for example, do not have the stringent requirements for microporosity and for access of electrolyte to a high surface area of solid sulfur-based material as in a solid sulfur-based cathode layer.
Despite the various approaches proposed for the fabrication of high energy density rechargeable cells comprising elemental sulfur, organo-sulfur, or carbon-sulfur polymer materials in a solid composite cathode, there remains a need for improved composite cathodes comprising electroactive sulfur materials, which have a combination of excellent access to the electrolyte and high electrochemical utilization while retaining or improving the desirable properties of electrical conductivity, mechanical strength, compressibility, and adhesion in solid composite cathodes utilizing electroactive sulfur materials, such as, elemental sulfur and carbon-sulfur polymer materials.